A stent is a metal coil or mesh tube that can be placed within a lumen, which can be a blood vessel, in order to provide support and/or to keep the lumen open. Stents may be implemented to treat a variety of medical conditions, for example, an aneurysm which is the dilation of a blood vessel resulting in stretching of the vessel wall, or a stenosis which is a partial occlusion of a blood vessel.
A conventional procedure for placing a stent includes the following sequence of steps. A guidewire is initially inserted at the point of entry, which is usually a small percutaneous incision in the arm or groin, and is then transferred through one or more blood vessels to the target site (e.g., a site defined at or near the aneurysm or the stenosis). Thereafter a hollow generally cylindrical catheter is slipped over the guidewire and directed to the target site by following the guidewire. The stent is generally compressed or compacted in order to facilitate its navigation through the catheter to the target site. Thereafter, the stent is expanded to support a localized region of the vessel wall and/or to keep the vessel open.
The stent must be precisely positioned at a predetermined location within the blood vessel (e.g., at the dilation or occlusion) in order to most effectively treat the underlying medical condition. Stent is maneuvered by sliding along the guidewire. Stent placement precision is related to the accuracy with which it is placed with respect to the target site. Fluoroscopic or other radiographic imaging can be used to track and navigate the guidewire and other tools (e.g. catheter, balloon, stent) to the deployment location.
After the stent is deployed in the vessel, it is desirable to confirm proper stent position before deployment and completion of the surgical procedure. The clinician can then take corrective action, e.g. re-inflate the balloon. However, most often the deployed stent is barely visible in X-ray images and must be enhanced with image processing techniques. Typical stent enhancement techniques consist in combining several images of the stent after motion compensation. The small size of the stent struts present one challenge, but a greater challenge is compensating for the movement of and within the patient. Currently available motion compensation techniques rely on detecting, across several images, the radiopaque markerballs that are attached to the delivery balloon. The delivery balloon is held in position relative to the deployed stent and the markerballs used to estimate and compensate for the stent motion.
This approach is most suited for checking the stent right after the stent has been deployed, because the delivery balloon is still in the artery and positioned relative to the stent. However, if the clinician wants to check the deployment of a stent that was implanted during a previous intervention, which may have occurred days, months, or years ago, the clinician must insert a new balloon in the artery on purpose in order to position the balloon markerballs needed to perform stent enhancement. Additionally, in the case of intra-procedure use of the markerballs of the delivery balloon, marker-based motion compensation has known limitations, e.g. the markerballs may move with respect to the stent by sliding along the guidewire due to the blood flow or improper balloon anchoring.